Optical device

ABSTRACT

An optical device comprises an optical filter having a substrate and a multilayer film having layers with different refractive indexes formed on at least one side of the substrate; and an infrared light emitting and receiving device having a first conductive-type semiconductor layer, an active layer, and a second conductive-type semiconductor layer. The multilayer film has alternatively stacked first second layers each having refractive indexes of 1.2 or more and 2.5 or less, and 3.2 or more and 4.2 or less, respectively, in a wavelength range of 2400 nm to 6000 nm. The optical filter includes a wavelength range having an average transmittance of 70% or more with a width of 50 nm or more in a wavelength range of 2400 nm to 6000 nm, and has a maximum transmittance of 5% or more in a wavelength range of 6000 nm to 8000 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 16/536,545 filed Aug. 9, 2019, which claimspriority of Japanese Patent Application No. 2018-160781 filed Aug. 29,2018, and Japanese Patent Application No. 2019-103829 filed Jun. 3,2019. The disclosures of the prior applications are hereby incorporatedby reference herein in their entirety.

TECHNICAL FIELD

This disclosure relates to a NDIR gas sensor and an optical device.

BACKGROUND

A non-dispersive infrared (NDIR) gas concentration measuring device hasbeen known as a gas concentration measuring device for measuring theconcentration of a gas to be measured in the atmosphere. Different kindsof gas absorb infrared light at different wavelengths, and anon-dispersive infrared absorption gas concentration measuring deviceutilizes this principle and measures the concentration of a gas bydetecting its absorption amount. Examples of the gas concentrationmeasuring device utilizing this principle include a device which isobtained by combining a filter (a transmission member) that transmitsinfrared light limited to a wavelength at which a gas to be measured hasabsorption characteristics and an infrared light receiving device, andis configured to measure the concentration of the gas to be measured bymeasuring the absorption amount of infrared light absorbed by the gas.JP H09-33431 A (PTL 1) describes a carbon dioxide concentrationmeasuring device utilizing this principle.

CITATION LIST Patent Literature

-   PTL 1: JP H09-33431 A

SUMMARY

However, an optimum combination of an infrared light source, an infraredlight emitting and receiving device and an optical filter for each gasto be detected has not been studied. In particular, the specificationsof the optical filter have not been optimized so far.

It could thus be helpful to provide a highly accurate NDIR gas sensorand a highly accurate optical device even using a simplified opticalfilter.

The primary features of this disclosure are as described below.

One aspect of the NDIR gas sensor of the present disclosure includes:

an optical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and

an infrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; where

the multilayer film has a structure in which a first layer and a secondlayer are alternately stacked, the first layer has a refractive index of1.2 or more and 2.5 or less in a wavelength range of 2400 nm to 6000 nm,and the second layer has a refractive index of 3.2 or more and 4.2 orless in a wavelength range of 2400 nm to 6000 nm;

the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) orInAs_(y)Sb_(1-y) (0.75≤y≤1); and

the optical filter includes a wavelength range having an averagetransmittance of 70% or more with a width of 50 nm or more in awavelength range of 2400 nm to 6000 nm, and has a maximum transmittanceof 5% or more in a wavelength range of 6000 nm to 8000 nm and an averagetransmittance of 2% or more and 60% or less in a wavelength range of6000 nm to 8000 nm.

Another aspect of the NDIR gas sensor of the present disclosureincludes:

an optical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and

an infrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; where

the multilayer film has a structure in which a first layer and a secondlayer are alternately stacked, the first layer contains at least oneselected from the group consisting of SiO, SiO₂, TiO₂ and ZnS, and thesecond layer contains at least one selected from the group consisting ofSi and Ge;

the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) orInAs_(y)Sb_(1-y) (0.75≤y≤1); and

the optical filter includes a wavelength range having an averagetransmittance of 70% or more with a width of 50 nm or more in awavelength range of 2400 nm to 6000 nm, and has a maximum transmittanceof 5% or more in a wavelength range of 6000 nm to 8000 nm and an averagetransmittance of 2% or more and 60% or less in a wavelength range of6000 nm to 8000 nm.

One aspect of the optical device of the present disclosure includes:

an optical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and

an infrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; where

the multilayer film has a structure in which a first layer and a secondlayer are alternately stacked, the first layer has a refractive index of1.2 or more and 2.5 or less in a wavelength range of 2400 nm to 6000 nm,and the second layer has a refractive index of 3.2 or more and 4.2 orless in a wavelength range of 2400 nm to 6000 nm;

the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) orInAs_(y)Sb_(1-y) (0.75≤y≤1); and

the optical filter includes a wavelength range having an averagetransmittance of 70% or more with a width of 50 nm or more in awavelength range of 2400 nm to 6000 nm, and has a maximum transmittanceof 5% or more in a wavelength range of 6000 nm to 8000 nm and an averagetransmittance of 2% or more and 60% or less in a wavelength range of6000 nm to 8000 nm.

Another aspect of the optical device of the present disclosure includes:

an optical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and

an infrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; where

the multilayer film has a structure in which a first layer and a secondlayer are alternately stacked, the first layer contains at least oneselected from the group consisting of SiO, SiO₂, TiO₂ and ZnS, and thesecond layer contains at least one selected from the group consisting ofSi and Ge;

the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) orInAs_(y)Sb_(1-y) (0.75≤y≤1); and

the optical filter includes a wavelength range having an averagetransmittance of 70% or more with a width of 50 nm or more in awavelength range of 2400 nm to 6000 nm, and has a maximum transmittanceof 5% or more in a wavelength range of 6000 nm to 8000 nm and an averagetransmittance of 2% or more and 60% or less in a wavelength range of6000 nm to 8000 nm.

Note that the above summary does not enumerate all the necessaryfeatures of the present disclosure. In addition, a subcombination ofthese features is also included in the present disclosure.

According to the present disclosure, it is possible to provide a highlyaccurate NDIR gas sensor and a highly accurate optical device even usinga simplified optical filter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates an example of the cross section of the optical filterin the present embodiment;

FIG. 2 illustrates an example of the NDIR gas sensor of the presentembodiment;

FIG. 3 illustrates and compares the optical filter of the presentdisclosure and the optical filter of the comparative example;

FIG. 4 illustrates the difference between the optical filter of thecomparative example and the optical filter of the present disclosure;

FIG. 5 illustrates the stacked structure of each layer of the infraredlight emitting and receiving device of Examples 1 to 3;

FIG. 6 illustrates the spectrum of the infrared light receiving deviceof Examples 1 and 3;

FIG. 7 illustrates and compares the transmission spectrum of the opticalfilter (simplified filter) of Examples 1 and 2 and the transmissionspectrum of the optical filter (conventionally configured filter) ofComparative Example;

FIG. 8 illustrates the stacked structure of the simplified filter ofExamples 1 and 2;

FIG. 9 illustrates the spectrum of an IR-sensor for CO₂ detection whichis configured by combining the infrared light receiving device and theoptical filter (simplified filter) of Example 1;

FIG. 10 illustrates the emission spectrum of the infrared light emittingdevice of Example 2;

FIG. 11 illustrates the emission spectrum of an IR-LED for CO₂ detectionwhich is configured by combining the infrared light emitting device andthe optical filter (simplified filter) of Example 2;

FIG. 12 illustrates and compares the transmission spectrum of theoptical filter (simplified filter) of Examples 3 and 5 and thetransmission spectrum of the optical filter (conventionally configuredfilter) of Comparative Example;

FIG. 13 illustrates the stacked structure of the simplified filter ofExamples 3 and 5;

FIG. 14 illustrates the spectrum of an IR-sensor for CO₂ detection whichis configured by combining the infrared light receiving device and theoptical filter (simplified filter) of Example 3;

FIG. 15 illustrates the stacked structure of each layer of the infraredlight emitting and receiving device of Example 4;

FIG. 16 illustrates the spectrum of the infrared light receiving deviceof Example 4;

FIG. 17 illustrates and compares the transmission spectrum of theoptical filter (simplified filter) of Example 4 and the transmissionspectrum of the optical filter (conventionally configured filter) ofComparative Example;

FIG. 18 illustrates the stacked structure of the simplified filter ofExample 4;

FIG. 19 illustrates the spectrum of an IR-sensor for CH₄ detection whichis configured by combining the infrared light receiving device and theoptical filter (simplified filter) of Example 4;

FIG. 20 illustrates the stacked structure of each layer of the infraredlight emitting and receiving device of Example 5;

FIG. 21 illustrates the spectrum of the infrared light receiving deviceof Example 5;

FIG. 22 illustrates the spectrum of an IR-sensor for CO₂ detection whichis configured by combining the infrared light receiving device and theoptical filter (simplified filter) of Example 5;

FIG. 23 illustrates the wavelength dispersion data of the refractiveindex of the dielectric material used for the optical filter; and

FIG. 24 illustrates the cut-off wavelength of the light emitting andreceiving component.

DETAILED DESCRIPTION

The following describes the present disclosure based on an embodiment ofthe present disclosure (hereinafter, referred to as “presentembodiment”). However, the scope of the claims is not limited by thefollowing embodiment.

In addition, not all combinations of features described in theembodiment are essential to the solution to the problem.

The following describes the present embodiment with reference to thedrawings. In the respective drawings described below, partscorresponding to each other are indicated by the same referencenumerals, and the repetition of explanation is suitably omitted.

The present embodiment simply provides an example for realizing thetechnical idea of the present disclosure, and does not specify thematerial, shape, structure, arrangement, dimensions, or the like of eachpart.

The present embodiment includes all combinations of features describedbelow.

The technical idea of the present disclosure may be variously modifiedwithout departing from the technical scope defined by the claims.

[NDIR Gas Sensor]

One aspect of the NDIR gas sensor of the present embodiment includes: anoptical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and an infrared light emitting and receivingdevice having a semiconductor layer of a first conductive type, anactive layer, and a semiconductor layer of a second conductive type;where the multilayer film has a structure in which a first layer and asecond layer are alternately stacked, the first layer has a refractiveindex of 1.2 or more and 2.5 or less in a wavelength range of 2400 nm to6000 nm, and the second layer has a refractive index of 3.2 or more and4.2 or less in a wavelength range of 2400 nm to 6000 nm; the activelayer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) or InAs_(y)Sb_(1-y)(0.75≤y≤1); and the optical filter includes a wavelength range having anaverage transmittance of 70% or more and 95% or less with a width of 50nm or more and 1000 nm or less in a wavelength range of 2400 nm to 6000nm, and has a maximum transmittance of 5% or more in a wavelength rangeof 6000 nm to 8000 nm.

Another aspect of the NDIR gas sensor of the present embodimentincludes: an optical filter having a substrate and a multilayer filmhaving a plurality of layers with different refractive indexes formed onat least one side of the substrate; and an infrared light emitting andreceiving device having a semiconductor layer of a first conductivetype, an active layer, and a semiconductor layer of a second conductivetype; where the multilayer film has a structure in which a first layerand a second layer are alternately stacked, the first layer contains atleast one selected from the group consisting of SiO, SiO₂, TiO₂ and ZnS,and the second layer contains at least one selected from the groupconsisting of Si and Ge; the active layer contains Al_(x)In_(1-x)Sb(0.02≤x≤0.20) or InAs_(y)Sb_(1-y) (0.75≤y≤1); and the optical filterincludes a wavelength range having an average transmittance of 70% ormore and 95% or less with a width of 50 nm or more and 1000 nm or lessin a wavelength range of 2400 nm to 6000 nm, and has a maximumtransmittance of 5% or more in a wavelength range of 6000 nm to 8000 nm.

FIG. 1 illustrates an example of the cross section of the optical filterin the present embodiment.

As illustrated in FIG. 1, the optical filter of this example has amultilayer film formed by alternately stacking a layer of alow-refractive index material (L) and a layer of a high-refractive indexmaterial (H) on both sides of a Si substrate, where the low-refractiveindex material (L) has a refractive index of 1.2 to 2.5 in thewavelength range of 2400 nm to 6000 nm, the high-refractive indexmaterial (H) has a refractive index of 3.2 to 4.2 in the wavelengthrange of 2400 nm to 6000 nm, and the layer directly provided on the Sisubstrate is of the high-refractive index material (H). Theconfiguration of the optical filter of the present embodiment is notlimited to the one illustrated in FIG. 1. In addition, the multilayerfilm may be formed only on one side of the substrate, or may be formedon both sides of the substrate.

For the optical filter of the present embodiment, the multilayer film isnot particularly limited as long as the first layer having a refractiveindex of 1.2 to 2.5 and the second layer having a refractive index of3.2 to 4.2 are alternately stacked. However, it is preferable that thehigh-refractive index material (H) be directly provided on the substrateas exemplified in FIG. 1, in order to enhance the effect of the presentdisclosure.

In the present embodiment, the optical filter is an interference typeband pass filter of the mid-infrared range. In general, an interferencetype band pass filter of the mid-infrared range has a large number ofstacked layers, which tends to cause more defects during film formation.In addition, a large number of stacked layers in the optical filter makeit difficult to miniaturize the NDIR gas sensor. Therefore, it ispreferable that the number of stacked layers of the optical filter besmall. However, simply reducing the number of stacked layers of theoptical filter for simplification may degrade the accuracy of the gassensor.

In order to provide a highly accurate NDIR gas sensor, it is alsonecessary to reduce the influence of absorption of infrared light bygases other than the gas to be detected.

As a result of examining an optimum combination of an infrared lightemitting and receiving device and an optical filter, we have realized aNDIR gas sensor where the accuracy would not be degraded even using asimplified optical filter, as described below.

The simplified optical filter here limits the blocking function in aregion without sensor sensitivity, thereby reducing the stacked layersof optical thin film used for blocking in this region. Using asimplified optical filter with a small number of stacked layers isexpected to improve the mass productivity. In addition, reducing thenumber of stacked layers can reduce the defects caused during filmformation, thereby improving the yield. Furthermore, warpage due to alarge number of stacked layers is reduced, which suppresses theoccurrence of chipping during dicing and enhances the mass productionstability. The region without sensor sensitivity is determined asfollows. The infrared light emitting and receiving device determines thecutoff wavelength on the long wavelength side based on the band gapenergy Eg (see FIG. 24), and the details will be described later. In thepresent embodiment, the band gap energy Eg corresponds to 6000 nm, andfor example, the wavelength range of 6000 nm to 8000 nm is cut off.Hereinafter, the region without sensor sensitivity in the presentembodiment is the wavelength range of 6000 nm to 8000 nm.

According to the NDIR gas sensor of the present embodiment, it ispossible to use an inexpensive optical filter that can be easilyproduced while maintaining the gas detection accuracy, even in the casewhere the active layer of the infrared light emitting and receivingdevice contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) or InAs_(y)Sb_(1-y)(0.75≤y≤1), and the optical filter includes a wavelength range having anaverage transmittance of 70% or more and 95% or less with a width of 50nm or more and 1000 nm or less in the wavelength range of 2400 nm to6000 nm and has a maximum transmittance of 5% or more in the wavelengthrange of 6000 nm to 8000 nm.

Examples of the gas to be detected include gas species having absorptioncharacteristics with respect to infrared light in a wavelength band of 2μm to 10 μm such as CO₂, CO, methane, H₂O, NO, C₂H₅OH, C₃H₈, NH₃, andCH₂O, but are not limited thereto.

The infrared light emitting and receiving device provided in the NDIRgas sensor of the present embodiment is an infrared light emittingdevice (for example, an infrared LED) and/or an infrared light receivingdevice (for example, an infrared photodiode). That is, the infraredlight emitting and receiving device is at least one of an infrared lightemitting device or an infrared light receiving device. The infraredlight receiving device is not limited to a photodiode, and can beselected from various devices such as a photoconductive sensor, athermistor, and a thermopile. The infrared light emitting device is notlimited to a light emitting diode, and can be selected from variousdevices such as a lamp and a MEMS heater. The infrared light emittingdevice and the infrared light receiving device preferably containAl_(x)In_(1-x)Sb (0.02≤x≤0.20) or InAs_(y)Sb_(1-y) (0.75≤y≤1) in theactive layer.

FIG. 2 illustrates an example of the NDIR gas sensor of the presentembodiment.

As illustrated in FIG. 2, in the NDIR gas sensor of the presentembodiment, an infrared light receiving device is disposed in theoptical path of infrared light to be emitted from an infrared lightemitting device, and an optical filter for selectively transmitting theabsorption wavelength of the gas to be detected is disposed in front ofthe infrared light receiving device.

FIG. 3 illustrates and compares the optical filter of the presentdisclosure and an optical filter of a comparative example.

The technique of the comparative example of FIG. 3 has no wavelengthselectivity in spectral sensitivity (the sensitivity does not changebased on wavelength). On the other hand, the present disclosure haswavelength selectivity as illustrated in FIG. 3. Therefore, in thepresent disclosure, the optical filter does not need to cut off thewavelength without sensitivity, which simplifies the function requiredfor the optical filter.

FIG. 4 illustrates the difference between the optical filter of thecomparative example and the optical filter of the present disclosure.

In the NDIR gas sensor of the present embodiment, the thickness of thefilm including a plurality of stacked layers can be significantlyreduced as illustrated in FIG. 4, where the plurality of stacked layersare called a cut surface and determine the cut characteristics. Asdescribed in the following Examples section, in the case of forming amultilayer film on both sides of a substrate, the ratio of the totalfilm thickness of each side can be included in the range of 0.5 to 2.0.That is, it is possible to make the film thickness of the cut surface,which is generally larger than twice the film thickness of the band passsurface, 0.5 to 2.0 times the film thickness of the band pass surface.

The following describes each component of the NDIR gas sensor of oneaspect of the present disclosure.

-Optical Filter-

The optical filter has a substrate, and a multilayer film having aplurality of layers with different refractive indexes formed on thesubstrate. The multilayer film may be formed only on one side of thesubstrate, or may be formed on both sides of the substrate.

The optical filter is disposed somewhere in the optical path of infraredlight emitted from the infrared light emitting device to the infraredlight receiving device inside the NDIR gas sensor. The optical filtermay be integrally formed with the infrared light emitting device, or maybe integrally formed with the infrared light receiving device. It may bedisposed in a predetermined position in the optical path. The NDIR gassensor may include a plurality of optical filters.

The optical filter can be prepared by depositing the first layer and thesecond layer on the substrate by vapor deposition.

-Substrate-

Since the multilayer film is formed on one side of the substrate, thesubstrate may be any substrate suitable for the formation of each layerconstituting the multilayer film. Examples thereof include, but are notlimited to, Si substrates, Ge substrates, ZnS substrates, and sapphiresubstrates.

-Multilayer Film-

The multilayer film is a film having a plurality of layers withdifferent refractive indexes, and specifically has a structure in whicha first layer and a second layer are alternately stacked, the firstlayer has a refractive index of 1.2 or more and 2.5 or less in thewavelength range of 2400 nm to 6000 nm, and the second layer has arefractive index of 3.2 or more and 4.2 or less in the wavelength rangeof 2400 nm to 6000 nm.

The optical filter of the present embodiment includes a wavelength rangehaving an average transmittance of 70% or more and 95% or less with awidth of 50 nm or more and 1000 nm or less in the wavelength range of2400 nm to 6000 nm, and has a maximum transmittance of 5% or more in thewavelength range of 6000 nm to 8000 nm.

Note that the width of the wavelength range having an averagetransmittance of 70% or more and 95% or less is based on the sum total,when there are two or more such wavelength ranges.

In the optical filter of the present embodiment, the averagetransmittance in the wavelength range of 6000 nm to 8000 nm ispreferably 2% or more and 60% or less, more preferably 30% or more and60% or less, and still more preferably 40% or more and 60% or less. Theoptical filter here has the above-described cut surface and band passsurface sandwiching a Si substrate (see FIG. 1). The upper limit of theaverage transmittance in the wavelength range of 6000 nm to 8000 nm is60%, which corresponds to the upper limit of the average transmittancein the same wavelength range of the band pass surface in the presentembodiment. In general, the transmittance of the band pass surface doesnot reach 100% even in the region without sensor sensitivity. Therefore,the average transmittance of the optical filter in the wavelength rangeof 6000 nm to 8000 nm has an upper limit of less than 100%, and theupper limit is determined by the average transmittance of the band passsurface. On the other hand, the lower limit of the average transmittanceof the optical filter in the wavelength range of 6000 nm to 8000 nmvaries depending on the average transmittance of the cut surface. Thatis, it is possible to change the lower limit of the averagetransmittance of the optical filter to, for example, 40%, 30% or 2%depending on the stacking state of the cut surface.

The reason why there is a suitable value for the average transmittancein the wavelength range of 6000 nm to 8000 nm is as follows. The numberof stacked layers of the optical filter tends to depend on the width ofthe wavelength range to be blocked. Because there are more than one gasabsorption (H₂O, SO₂, C₃H₈, C₂H₅OH, CH₂O, CH₄, NO, etc.) in the range of6000 nm to 8000 nm, it is necessary to block the mid-infrared light inthe wavelength range of 6000 nm to 8000 nm particularly for a NDIR gassensor so that a specific gas can be detected with high accuracy.

On the other hand, the sensitivity range of the semiconductor lightemitting and receiving component affects the state density and theBoltzmann distribution. FIG. 24 illustrates the cut-off wavelength ofthe semiconductor light emitting and receiving component. In particular,the rising wavelength of the sensitivity on the long wavelength sidedepends on the band gap energy Eg. The present disclosure can provide asensor that has sensitivity in a specific absorption wavelength rangeaccording to the gas to be detected while having no sensitivity in thewavelength range of 6000 nm to 8000 nm, by optimally designing the bandgap energy of the light emitting and receiving component. As a result,the blocking of 6000 nm to 8000 nm by an optical filter is no longerimportant, which simplifies the design of the optical filter, reducesthe cost, and improves the mass productivity. In this case, the averagetransmittance in the wavelength range of 6000 nm to 8000 nm is more than2%, and when the average transmittance is in the range of 2% to 60%, itis possible to simplify the optical filter while maintaining theperformance as a NDIR gas sensor.

On the other hand, in order to provide a highly accurate gas sensor, itis desirable that the shape of a transmission spectrum having an averagetransmittance of 70% or more included in the wavelength range of 2400 nmto 6000 nm be close to a rectangle. In order to make the rising andfalling shape of the transmission spectrum into a sharp shape that isclose to a rectangular, the number of stacked layers of the opticalfilter is increased, and this leads to a certain blocking function evenin the wavelength range of 6000 nm to 8000 nm. Therefore, the averagetransmittance is set within 60%. That is, the upper and lower limits inthe wavelength range of 6000 nm to 8000 nm indicate a range in which theoptical filter can be simplified while maintaining the performance as aNDIR gas sensor.

(Method of Measuring Average Transmittance of Optical Filter)

A method of confirming that the average transmittance of the opticalfilter in the wavelength range of 2400 nm to 6000 nm is 70% or more and95% or less includes obtaining a transmission spectrum, for example, ina wave number range of 900 cm⁻¹ to 4200 cm⁻¹ with a wave numberresolving power of 8 cm⁻¹ by a microscopic FT-IR apparatus (Hyperion3000+TENSOR 27 made by Bruker), and dividing the numerical integralvalue of the transmittance in the above wavelength range by thewavelength range (range). The number of measurement points is 200 pointsper 1000 nm (one point per 5 nm). The average transmittance in thewavelength range of 6000 nm to 8000 nm can also be confirmed with thesame method.

In the case of stacking more than one first layer, the material and thethickness of each first layer may be the same or different. The sameapplies to the second layer. The multilayer film may further includelayers other than the first layer and the second layer. Even in the casewhere the first layer and the second layer are alternately stacked, itis possible to have a stacked structure such as a first layer→a secondlayer→a third layer→a first layer→a second layer . . . .

The thickness of the multilayer film is preferably 5000 nm or more and25000 nm or less, and more preferably 5000 nm or more and 20000 nm orless. In this way, the time for optical filter production can beshortened and the yield can be improved. The film thickness can bemeasured by cross sectional SEM observation.

Taking one first layer and one second layer as one repeating unit, thenumber of times of alternately stacking the first layer and the secondlayer, i.e. the number of repeating units, is preferably 10 or more and60 or less and more preferably 10 or more and 40 or less, respectively.In this way, the time for optical filter production can be shortened andthe yield can be improved.

The number of times of alternately stacking can be measured by crosssectional SEM observation.

-First Layer-

The first layer of the multilayer film of the present embodiment is alayer having a refractive index of 1.2 or more and 2.5 or less in thewavelength range of 2400 nm to 6000 nm.

Examples of specific materials of the first layer include SiO, SiO₂,TiO₂, and ZnS. The first layer may be made of the above materials.

-Second Layer-

The second layer of the multilayer film of the present embodiment is alayer having a refractive index of 3.2 or more and 4.2 or less in thewavelength range of 2400 nm to 6000 nm.

Examples of specific materials of the second layer include Si and Ge.The second layer may be made of the above materials.

(Method of Measuring Refractive Index of First Layer and Second Layer)

Note that the refractive index in the present embodiment is a valuemeasured by an ellipsometer in accordance with JIS K7142.

-Infrared Light Emitting and Receiving Device-

The infrared light emitting and receiving device includes asemiconductor layer of a first conductive type, an active layer ofAl_(x)In_(1-x)Sb (0.02≤x≤0.20) or InAs_(y)Sb_(1-y) (0.75≤y≤1), and asemiconductor layer of a second conductive type.

The expression “light emitting and receiving” means having at least oneof light emitting function and light receiving function, and theinfrared light emitting and receiving device specifically refers to aninfrared light emitting diode and an infrared photodiode.

In addition, the expression “containing Al_(x)In_(1-x)Sb (0.02≤x≤0.20)”means that Al, In and Sb are contained in the layer, but this expressionalso includes cases of containing other elements. Specifically, thisexpression also includes the case where a slight change is made to thecomposition of this layer, for example, by adding a small amount ofother elements (for example, elements such as As, P, Ga, and N in notmore than several percent). The term “containing” used in expressing thecomposition of other layers has the same meaning.

(Method of Measuring Al Composition of Each Layer)

The Al composition of each layer was determined as follows by SecondaryIon Mass Spectrometry (SIMS) method. A magnetic field type SIMSapparatus IMS 7f made by CAMECA was used for the measurement. In thismethod, compositional analysis is performed by irradiating a solidsurface with beam type primary ion species, digging in the depthdirection by means of sputtering phenomenon, and simultaneouslydetecting the generated secondary ions. The Al composition here refersto the ratio of Al element to all 13 group elements contained in eachlayer.

Specifically, cesium ion (Cs+) was used as the primary ion species, theprimary ion energy was set to 2.5 keV, and the beam incident angle wasset to 67.2°. Under these conditions, MCs+ (M is Al, Ga, In, As, Sb, orthe like) with a small matrix effect was detected as the secondary ionspecies to be detected.

At this time, sputtering was carried out under the above-describedconditions and up to the depth of the target layer for a predeterminedperiod of time to analyze the composition of the target layer. The depthof the target layer can be obtained from the thickness of each layer bycross sectional TEM measurement as described later. For the sputteringtime-depth conversion in SIMS analysis, sputtering rate was obtained bymeasuring the sputtering depth for a certain period of time under thesame condition as the analysis with, for example, a stylus profilometer,and the obtained sputtering rate was used to convert the sputtering timein the sample measurement into depth.

Then, the Al composition was obtained from the signal intensity of MCs+in each layer. For example, in the case of an AlInSb layer, the Alcomposition was obtained by: (signal intensity of AlCs+)/((signalintensity of AlCs+)+(signal intensity of InCs+)).

Even if each layer has a uniform composition in the depth direction, thesignal intensity sometimes distributes in the depth direction due to theinfluence of sputtering. In this case, the signal intensity of eachlayer is represented by the maximum signal intensity.

Note that the quantitative value of the Al composition obtained by theanalysis can be accompanied by deviation from the true value. In orderto correct this deviation from the true value, a separate sample forwhich the lattice constant value had been obtained with the X-raydiffraction (XRD) method was prepared, and, using this sample as astandard sample with a known Al composition value, SIMS analysis wasperformed under the conditions for measuring the Al composition of eachlayer. In this way, the sensitivity coefficient of the Al compositionwith respect to the signal intensity was obtained. The Al composition ofeach layer was obtained by multiplying the SIMS signal intensity by thesensitivity coefficient.

In this case, Al_(x)In_(1-x)Sb having a film thickness of 800 nm stackedon a GaAs substrate was used as the separate sample. For this sample,the lattice constant was obtained with the X-ray diffraction (XRD)method using an X-ray diffractometer X′Pert MPD made by Spectris Co.,Ltd., as described below, to obtain the Al composition x of the standardsample.

By performing 2θ-ω scan by X-ray diffraction, the lattice constant inthe direction normal to the substrate surface of the layer containingAl_(x)In_(1-x)Sb was obtained from the peak position in the 2θ-ω scan ofthe plane index of the plane corresponding to the plane orientation ofthe substrate surface, and the Al composition x was determined from thelattice constant in the normal direction using the Vegard's rule. Inthis case, it is assumed that there is no anisotropic distortion of theAl_(x)In_(1-x)Sb layer. The Vegard's rule is specifically representedby:

a _(AlInSb) =xa _(AlSb)+(1−x)a _(InSb)  Expression (1)

Where a_(AlSb) is the lattice constant of AlSb, a_(InSb) is the latticeconstant of InSb, and a_(AlInSb) is the lattice constant of theAl_(x)In_(1-x)Sb obtained by the above-described X-ray diffraction. Inaddition, 6.1355 Å was used for a_(AlSb), and 6.4794 Å was used fora_(InSb). A sample with 0.10<x<0.15 was used as a standard sample forSIMS measurement.

The As composition y in the case where the active layer containsInAs_(y)Sb_(1-y) (0.75≤y≤1) can also be measured with the same method asdescribed above. In this case, InAs_(y)Sb_(1-y) having a film thicknessof 800 nm stacked on a GaAs substrate was used as the separate sample,and the Vegard's rule is specifically represented by:

a _(InAsSb) =ya _(InAs)+(1−y)a _(InSb)  Expression (2)

Where a_(InAs) is the lattice constant of InAs, a_(InSb) is the latticeconstant of InSb, and a_(InAsSb) is the lattice constant of theInAs_(y)Sb_(1-y) obtained by the above-described X-ray diffraction. Inaddition, 6.0585 Å was used for a_(InAs), and 6.4794 Å was used fora_(InSb). A sample with 0.10<y<0.15 was used as a standard sample forSIMS measurement.

The first conductivity type and the second conductivity type may be anyof n-type (including n-type impurities), i-type (including noimpurities) and p-type (including p-type impurities), respectively.

The semiconductor layer of the first conductivity type, the activelayer, and the semiconductor layer of the second conductivity type maybe formed on a semiconductor substrate such as a GaAs substrate or a Sisubstrate.

In the present embodiment, each layer may be provided in an order of thesubstrate, the semiconductor layer of the first conductivity type, theactive layer, and the semiconductor layer of the second conductivitytype, or may be provided in an order of the substrate, the semiconductorlayer of the second conductivity type, the active layer, and thesemiconductor layer of the first conductivity type. In the presentembodiment, it is preferable that the first conductivity type be n-typeand the second conductivity type be p-type.

In addition, in the present embodiment, one or more barrier layers maybe provided between the semiconductor layer of the first conductivitytype and the active layer and/or between the active layer and thesemiconductor layer of the second conductivity type, respectively.

In the present embodiment, it is preferable that an n-type barrier layerbe provided between the semiconductor layer of the first conductivitytype and the active layer and a p-type barrier layer be provided betweenthe active layer and the semiconductor layer of the second conductivitytype, respectively.

The n-type barrier layer is preferably n-type Al_(x)In_(1-x)Sb(0.20≤x≤0.35) so that the effect of the present disclosure can beimproved.

The p-type barrier layer is preferably p-type Al_(x)In_(1-x)Sb(0.20≤x≤0.35) so that the effect of the present disclosure can beimproved.

[Relationship Between Composition of Active Layer and Characteristics ofOptical Filter]

The preferred composition of the active layer and the characteristics ofthe optical filter with respect to each gas to be detected are asfollows.

In the case where the gas to be detected is NH₃ (having absorption in5500 nm to 6500 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.02≤x≤0.05) or InAs_(y)Sb_(1-y) (0.75≤y≤0.90), andthe optical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 400 nm or less in a wavelength range of 5600 nm to 6000 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

In the case where the gas to be detected is NO (having absorption in5100 nm to 5700 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.02≤x≤0.05) or InAs_(y)Sb_(1-y) (0.75≤y≤0.90), andthe optical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 300 nm or less in a wavelength range of 5200 nm to 5500 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

In the case where the gas to be detected is CO (having absorption in4400 nm to 5000 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.02≤x≤0.12) or InAs_(y)Sb_(1-y) (0.75≤y≤0.90), andthe optical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 300 nm or less in a wavelength range of 4500 nm to 4800 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

In the case where the gas to be detected is CO₂ (having absorption in4100 nm to 4400 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.02≤x≤0.12) or InAs_(y)Sb_(1-y) (0.75≤y≤0.90), andthe optical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 150 nm or less in a wavelength range of 4200 nm to 4350 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

In the case where the gas to be detected is CH₂O (having absorption in3100 nm to 3800 nm), or C₃H₈ (having absorption in 3200 nm to 3700 nm),or C₂H₅OH (having absorption in 3200 nm to 3700 nm), it is preferablethat the active layer include Al_(x)In_(1-x)Sb (0.04≤x≤0.14) orInAs_(y)Sb_(1-y) (0.80≤y≤1), and the optical filter include a wavelengthrange having an average transmittance of 70% or more and 95% or lesswith a width of 50 nm or more and 300 nm or less in a wavelength rangeof 3300 nm to 3600 nm and have a maximum transmittance of 5% or more ina wavelength range of 6000 nm to 8000 nm.

In the case where the gas to be detected is CH₄ (having absorption in3200 nm to 3500 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.04≤x≤0.14) or InAs_(y)Sb_(1-y) (0.80≤y≤1), and theoptical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 200 nm or less in a wavelength range of 3200 nm to 3400 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

In the case where the gas to be detected is H₂O (having absorption in2500 nm to 2900 nm), it is preferable that the active layer includeAl_(x)In_(1-x)Sb (0.08≤x≤0.20) or InAs_(y)Sb_(1-y) (0.80≤y≤1), and theoptical filter include a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 400 nm or less in a wavelength range of 2400 nm to 2800 nm andhave a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.

[Optical Device]

The optical device of the present embodiment has the same features asthe NDIR gas sensor of the present embodiment described above. Theoptical device is not limited to a NDIR gas sensor, and may be aninfrared radiation thermometer, an infrared spectral imaging, or a humanbody detection sensor having similar features.

According to the optical device of the present embodiment, it ispossible to obtain an effect of only selectively receiving/emittinginfrared light in a desired wavelength band, even using a simplifiedoptical filter.

One aspect of the optical device of the present embodiment includes: anoptical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and an infrared light emitting and receivingdevice having a semiconductor layer of a first conductive type, anactive layer, and a semiconductor layer of a second conductive type;where the multilayer film has a structure in which a first layer and asecond layer are alternately stacked, the first layer has a refractiveindex of 1.2 or more and 2.5 or less in a wavelength range of 2400 nm to6000 nm, and the second layer has a refractive index of 3.2 or more and4.2 or less in a wavelength range of 2400 nm to 6000 nm; the activelayer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.20) or InAs_(y)Sb_(1-y)(0.75≤y≤1); and the optical filter includes a wavelength range having anaverage transmittance of 70% or more and 95% or less with a width of 50nm or more and 1000 nm or less in a wavelength range of 2400 nm to 6000nm and has a maximum transmittance of 5% or more in a wavelength rangeof 6000 nm to 8000 nm.

Another aspect of the optical device of the present embodiment includes:an optical filter having a substrate and a multilayer film having aplurality of layers with different refractive indexes formed on at leastone side of the substrate; and an infrared light emitting and receivingdevice having a semiconductor layer of a first conductive type, anactive layer, and a semiconductor layer of a second conductive type;where the multilayer film has a structure in which a first layer and asecond layer are alternately stacked, the first layer contains at leastone selected from the group consisting of SiO, SiO₂, TiO₂ and ZnS, andthe second layer contains at least one selected from the groupconsisting of Si and Ge; the active layer contains Al_(x)In_(1-x)Sb(0.02≤x≤0.20) or InAs_(y)Sb_(1-y) (0.75≤y≤1); and the optical filterincludes a wavelength range having an average transmittance of 70% ormore and 95% or less with a width of 50 nm or more and 1000 nm or lessin a wavelength range of 2400 nm to 6000 nm and has a maximumtransmittance of 5% or more in a wavelength range of 6000 nm to 8000 nm.

EXAMPLES

The following describes the present disclosure in detail based onExamples. However, the present disclosure is not limited to thefollowing Examples, and may be variously modified without departing fromthe spirit of the disclosure.

Example 1

The following describes an IR-sensor obtained by combining a simplifiedfilter and an infrared light receiving device. The IR-sensor forms partof an optical device such as a CO₂ sensor. First, a PIN diode structurewas prepared with the MBE method. The active layer wasAl_(0.04)In_(0.96)Sb. The n-type semiconductor layer was doped with Snat 1.0×10¹⁹ atoms/cm³, so that the energy band was degenerated and itwas transparent to infrared light having a wavelength longer than 2000nm. In addition, n-type Al_(0.22)In_(0.78)Sb and p-typeAl_(0.22)In_(0.78)Sb were provided as barrier layers so as to sandwichthe active layer. FIG. 5 illustrates the stacked structure of each layerof the infrared light emitting and receiving device of Example 1.

A positive photoresist for i-line was coated on the surface of thesemiconductor wafer thus prepared, and exposure was performed using thei-line with a reduction projection type exposure machine. Subsequently,development was performed, and a plurality of resist patterns wereregularly formed on the surface of the semiconductor stacked portion.Subsequently, a plurality of mesas were formed by dry etching. Afterdepositing SiO₂ as a hard mask on the device having a mesa shape, deviceisolation was performed by dry etching, and then SiN was deposited as aprotective film, and a contact hole was formed by photolithography anddry etching. Subsequently, a plurality of mesas were connected in seriesby photolithography and sputtering. Thereafter, a polyimide resin wascovered on the device surface as a protective film.

The wafer prepared by the above-described pre-process was diced intopieces, connected to a lead frame using Au bonding wires, and sealedwith an epoxy mold resin so that a light receiving surface was exposed.The infrared light receiving device thus prepared was subjected tospectral sensitivity spectrum measurement, and the result wasillustrated in FIG. 6. FIG. 6 illustrates the spectrum of the infraredlight receiving device of Example 1. This sensor is sensitive toinfrared light around 4300 nm, which is the absorption band of CO₂, buthardly sensitive to infrared light longer than 6000 nm.

The optical filter was designed based on simulation. The simulationmethod was a known calculation method using the Fresnel coefficient. Inaddition, Ge and SiO were assumed as materials of the optical thin film,and literature values were used as the wavelength dispersion data of thecomplex refractive indexes. The design of the simplified filtercalculated by optical simulation is described below.

FIG. 7 illustrates and compares the transmission spectrum of the opticalfilter (simplified filter) of Example 1 and the transmission spectrum ofan optical filter (conventionally configured filter) of ComparativeExample. FIG. 8 illustrates the stacked structure of the simplifiedfilter of Example 1. As illustrates in FIG. 7, the simplified filter hasa transmission range around 2000 nm to 2500 nm, and 6500 nm or longer.Blocking function is unnecessary in this range, and thereby thethickness of the optical thin film can be reduced to 8432 nm as comparedwith 25000 nm of a conventional filter. The simplified filterillustrated in FIG. 7 has an average transmittance of 81% in thewavelength range of 4180 nm to 4330 nm. In addition, the maximumtransmittance in the wavelength range of 6000 nm to 8000 nm is 51%, andthe average transmittance in this wavelength range is 20%.

Next, FIG. 9 illustrates the spectrum of an IR-sensor for CO₂ detectionwhich is configured by combining the infrared light receiving device andthe optical filter (simplified filter) of Example 1. As the resultindicates, the output when combined with the infrared light receivingdevice is equivalent regardless of which optical filter is used. Inother words, it is indicated that the design of the optical filter canbe greatly simplified without any deterioration in the performance as asensor component for a NDIR type gas sensor.

Example 2

The following describes an IR-LED obtained by combining a simplifiedfilter and an infrared light emitting device. The IR-LED forms part ofan optical device such as a CO₂ sensor. First, a PIN diode structure wasprepared with the MBE method. The active layer was Al_(0.04)In_(0.96)Sb.The n-type semiconductor layer was doped with Sn at 1.0×10¹⁹ atoms/cm³,so that the energy band was degenerated and it was transparent toinfrared light having a wavelength longer than 2000 nm. In addition,n-type Al_(0.22)In_(0.78)Sb and p-type Al_(0.22)In_(0.78)Sb wereprovided as barrier layers so as to sandwich the active layer. FIG. 5illustrates the stacked structure of each layer of the infrared lightemitting and receiving device of Example 2.

A positive photoresist for i-line was coated on the surface of thesemiconductor wafer thus prepared, and exposure was performed using thei-line with a reduction projection type exposure machine. Subsequently,development was performed, and a plurality of resist patterns wereregularly formed on the surface of the semiconductor stacked portion.Subsequently, a plurality of mesas were formed by dry etching. Afterdepositing SiO₂ as a hard mask on the device having a mesa shape, deviceisolation was performed by dry etching, and then SiN was deposited as aprotective film, and a contact hole was formed by photolithography anddry etching. Subsequently, a plurality of mesas were connected in seriesby photolithography and sputtering. Thereafter, a polyimide resin wascovered on the device surface as a protective film.

The wafer prepared by the above-described pre-process was diced intopieces, connected to a lead frame using Au bonding wires, and sealedwith an epoxy mold resin so that a light emitting surface was exposed.The infrared light emitting device thus prepared was subjected toemission spectrum measurement, and the result was illustrated in FIG.10. FIG. 10 illustrates the emission spectrum of the infrared lightemitting device of Example 2. This IR-LED emits light in the infraredrange around 4300 nm, which is the absorption band of CO₂, but does notemit light in the infrared range longer than 6000 nm.

The optical filter was designed based on simulation. The simulationmethod was a known calculation method using the Fresnel coefficient. Inaddition, Ge and SiO were assumed as materials of the optical thin film,and literature values were used as the wavelength dispersion data of thecomplex refractive indexes. The design of the simplified filtercalculated by optical simulation is described below.

FIG. 7 illustrates and compares the transmission spectrum of the opticalfilter (simplified filter) of Example 2 and the transmission spectrum ofthe optical filter (conventionally configured filter) of ComparativeExample. FIG. 8 illustrates the stacked structure of the simplifiedfilter of Example 2. As illustrates in FIG. 7, the simplified filter hasa transmission range around 2000 nm to 2500 nm, and 6500 nm or longer.Blocking function is unnecessary in this range, and thereby thethickness of the optical thin film can be reduced to 8432 nm as comparedwith 25000 nm of a conventional filter. The simplified filterillustrated in FIG. 7 has an average transmittance of 81% in thewavelength range of 4180 nm to 4330 nm. In addition, the maximumtransmittance in the wavelength range of 6000 nm to 8000 nm is 51%, andthe average transmittance in this wavelength range is 20%.

Next, FIG. 11 illustrates the emission spectrum of an IR-LED for CO₂detection which is configured by combining the infrared light emittingdevice and the optical filter (simplified filter) of Example 2. As theresult indicates, the output when combined with the infrared lightemitting device is equivalent regardless of which optical filter isused. In other words, it is indicated that the design of the opticalfilter can be greatly simplified without any deterioration in theperformance as a light source for a NDIR type gas sensor.

Example 3

The following describes an IR-sensor obtained by combining a simplifiedfilter and an infrared light receiving device. The IR-sensor forms partof an optical device such as a CO₂ sensor. First, a PIN diode structurewas prepared with the MBE method. The active layer wasAl_(0.04)In_(0.96)Sb. The n-type semiconductor layer was doped with Snat 1.0×10¹⁹ atoms/cm³, so that the energy band was degenerated and itwas transparent to infrared light having a wavelength longer than 2000nm. In addition, n-type Al_(0.22)In_(0.78)Sb and p-typeAl_(0.22)In_(0.78)Sb were provided as barrier layers so as to sandwichthe active layer. FIG. 5 illustrates the stacked structure of each layerof the infrared light emitting and receiving device of Example 3.

A positive photoresist for i-line was coated on the surface of thesemiconductor wafer thus prepared, and exposure was performed using thei-line with a reduction projection type exposure machine. Subsequently,development was performed, and a plurality of resist patterns wereregularly formed on the surface of the semiconductor stacked portion.Subsequently, a plurality of mesas were formed by dry etching. Afterdepositing SiO₂ as a hard mask on the device having a mesa shape, deviceisolation was performed by dry etching, and then SiN was deposited as aprotective film, and a contact hole was formed by photolithography anddry etching. Subsequently, a plurality of mesas were connected in seriesby photolithography and sputtering. Thereafter, a polyimide resin wascovered on the device surface as a protective film.

The wafer prepared by the above-described pre-process was diced intopieces, connected to a lead frame using Au bonding wires, and sealedwith an epoxy mold resin so that a light receiving surface was exposed.The infrared light receiving device thus prepared was subjected tospectral sensitivity spectrum measurement, and the result wasillustrated in FIG. 6. FIG. 6 illustrates the spectrum of the infraredlight receiving device of Example 1. This sensor is sensitive toinfrared light around 4300 nm, which is the absorption band of CO₂, buthardly sensitive to infrared light longer than 6000 nm.

The optical filter was designed based on simulation. The simulationmethod was a known calculation method using the Fresnel coefficient. Inaddition, Si and SiO₂ were assumed as materials of the optical thinfilm, and literature values were used as the wavelength dispersion dataof the complex refractive indexes. The design of the simplified filtercalculated by optical simulation is described below.

FIG. 12 illustrates and compares the transmission spectrum of theoptical filter (simplified filter) of Example 3 and the transmissionspectrum of the optical filter (conventionally configured filter) ofComparative Example. FIG. 13 illustrates the stacked structure of thesimplified filter of Example 3. As illustrates in FIG. 12, thesimplified filter has a transmission range around 2000 nm to 2500 nm,and 6500 nm to 8000 nm. Blocking function is unnecessary in this range,and thereby the thickness of the optical thin film can be reduced to9780 nm as compared with 25000 nm of a conventional filter. Thesimplified filter illustrated in FIG. 12 has an average transmittance of76% in the wavelength range of 4180 nm to 4330 nm. In addition, themaximum transmittance in the wavelength range of 6000 nm to 8000 nm is62%, and the average transmittance in this wavelength range is 24%.

Next, FIG. 14 illustrates the spectrum of an IR-sensor for CO₂ detectionwhich is configured by combining the infrared light receiving device andthe optical filter (simplified filter) of Example 3. As the resultindicates, the output when combined with the infrared light receivingdevice is equivalent regardless of which optical filter is used. Inother words, it is indicated that the design of the optical filter canbe greatly simplified without any deterioration in the performance as asensor component for a NDIR type gas sensor.

Example 4

The following describes an IR-sensor obtained by combining a simplifiedfilter and an infrared light receiving device. The IR-sensor forms partof an optical device such as a CH₄ sensor. First, a PIN diode structurewas prepared with the MBE method. The active layer wasAl_(0.09)In_(0.91)Sb. The n-type semiconductor layer was doped with Snat 1.0×10¹⁹ atoms/cm³, so that the energy band was degenerated and itwas transparent to infrared light having a wavelength longer than 2000nm. In addition, n-type Al_(0.30)In_(0.70)Sb and p-typeAl_(0.30)In_(0.70)Sb were provided as barrier layers so as to sandwichthe active layer. FIG. 15 illustrates the stacked structure of eachlayer of the infrared light emitting and receiving device of Example 4.

A positive photoresist for i-line was coated on the surface of thesemiconductor wafer thus prepared, and exposure was performed using thei-line with a reduction projection type exposure machine. Subsequently,development was performed, and a plurality of resist patterns wereregularly formed on the surface of the semiconductor stacked portion.Subsequently, a plurality of mesas were formed by dry etching. Afterdepositing SiO₂ as a hard mask on the device having a mesa shape, deviceisolation was performed by dry etching, and then SiN was deposited as aprotective film, and a contact hole was formed by photolithography anddry etching. Subsequently, a plurality of mesas were connected in seriesby photolithography and sputtering. Thereafter, a polyimide resin wascovered on the device surface as a protective film.

The wafer prepared by the above-described pre-process was diced intopieces, connected to a lead frame using Au bonding wires, and sealedwith an epoxy mold resin so that a light receiving surface was exposed.The infrared light receiving device thus prepared was subjected tospectral sensitivity spectrum measurement, and the result wasillustrated in FIG. 16. FIG. 16 illustrates the spectrum of the infraredlight receiving device of Example 4. This sensor is sensitive toinfrared light around 3300 nm, which is the absorption band of CH₄, buthardly sensitive to infrared light longer than 6000 nm.

The optical filter was designed based on simulation. The simulationmethod was a known calculation method using the Fresnel coefficient. Inaddition, Ge and SiO were assumed as materials of the optical thin film,and literature values were used as the wavelength dispersion data of thecomplex refractive indexes. The design of the simplified filtercalculated by optical simulation is described below.

FIG. 17 illustrates and compares the transmission spectrum of theoptical filter (simplified filter) of Example 4 and the transmissionspectrum of the optical filter (conventionally configured filter) ofComparative Example. FIG. 18 illustrates the stacked structure of thesimplified filter of Example 4. As illustrated in FIG. 17, thesimplified filter has a transmission range at 6000 nm to 8000 nm.Blocking function is unnecessary in this range, and thereby thethickness of the optical thin film can be reduced to 7529 nm as comparedwith 25000 nm of a conventional filter. The simplified filterillustrated in FIG. 17 has an average transmittance of 78% in thewavelength range of 3260 nm to 3380 nm. In addition, the maximumtransmittance in the wavelength range of 6000 nm to 8000 nm is 85%, andthe average transmittance in this wavelength range is 52%.

Next, FIG. 19 illustrates the spectrum of an IR-sensor for CH₄ detectionwhich is configured by combining the infrared light receiving device andthe optical filter (simplified filter) of Example 4. As the resultindicates, the output when combined with the infrared light receivingdevice is equivalent regardless of which optical filter is used. Inother words, it is indicated that the design of the optical filter canbe greatly simplified without any deterioration in the performance as asensor component for a NDIR type gas sensor.

Example 5

The following describes an IR-sensor obtained by combining a simplifiedfilter and an infrared light receiving device. The IR-sensor forms partof an optical device such as a CO₂ sensor. First, a PIN diode structurewas prepared with the MBE method. The active layer wasInAs_(0.87)Sb_(0.13), and the n-type semiconductor layer degenerated theenergy band and was transparent to infrared light having a wavelengthlonger than 2000 nm. In addition, n-type Al_(0.30)In_(0.70)AsSb andp-type Al_(0.30)In_(0.70)AsSb were provided as barrier layers so as tosandwich the active layer. FIG. 20 illustrates the stacked structure ofeach layer of the infrared light emitting and receiving device ofExample 5.

A positive photoresist for i-line was coated on the surface of thesemiconductor wafer thus prepared, and exposure was performed using thei-line with a reduction projection type exposure machine. Subsequently,development was performed, and a plurality of resist patterns wereregularly formed on the surface of the semiconductor stacked portion.Subsequently, a plurality of mesas were formed by dry etching. Afterdepositing SiO₂ as a hard mask on the device having a mesa shape, deviceisolation was performed by dry etching, and then SiN was deposited as aprotective film, and a contact hole was formed by photolithography anddry etching. Subsequently, a plurality of mesas were connected in seriesby photolithography and sputtering. Thereafter, a polyimide resin wascovered on the device surface as a protective film.

The wafer prepared by the above-described pre-process was diced intopieces, connected to a lead frame using Au bonding wires, and sealedwith an epoxy mold resin so that a light receiving surface was exposed.The infrared light receiving device thus prepared was subjected tospectral sensitivity spectrum measurement, and the result wasillustrated in FIG. 21. FIG. 21 illustrates the spectrum of the infraredlight receiving device of Example 5. This sensor is sensitive toinfrared light around 4300 nm, which is the absorption band of CO₂, buthardly sensitive to infrared light longer than 6000 nm.

The optical filter was designed based on simulation. The simulationmethod was a known calculation method using the Fresnel coefficient. Inaddition, Si and SiO₂ were assumed as materials of the optical thinfilm, and literature values were used as the wavelength dispersion dataof the complex refractive indexes. The design of the simplified filtercalculated by optical simulation is described below.

FIG. 12 illustrates and compares the transmission spectrum of theoptical filter (simplified filter) of Example 5 and the transmissionspectrum of the optical filter (conventionally configured filter) ofComparative Example. FIG. 13 illustrates the stacked structure of thesimplified filter of Example 5. As illustrates in FIG. 12, thesimplified filter has a transmission range around 2000 nm to 2500 nm,and 6000 nm to 8000 nm. Blocking function is unnecessary in this range,and thereby the thickness of the optical thin film can be reduced to9780 nm as compared with 25000 nm of a conventional filter. Thesimplified filter illustrated in FIG. 12 has an average transmittance of76% in the wavelength range of 4180 nm to 4330 nm. In addition, themaximum transmittance in the wavelength range of 6000 nm to 8000 nm is62%, and the average transmittance in this wavelength range is 24%.

Next, FIG. 22 illustrates the spectrum of an IR-sensor for CO₂ detectionwhich is configured by combining the infrared light receiving device andthe optical filter (simplified filter) of Example 5. As the resultindicates, the output when combined with the infrared light receivingdevice is equivalent regardless of which optical filter is used. Inother words, it is indicated that the design of the optical filter canbe greatly simplified without any deterioration in the performance as asensor component for a NDIR type gas sensor.

The wavelength dispersion data of the refractive index of the dielectricmaterial used in the optical simulation of Examples 1 to 5 areillustrated in FIG. 23.

Detailed conditions and results of Examples and Comparative Example arelisted in Table 1. The wavelength-transmittance curves of Examples 1 and2 and Comparative Example are illustrated in FIG. 7, thewavelength-transmittance curves of Examples 3 and 5 are illustrated inFIG. 12, and the wavelength-transmittance curve of Example 4 isillustrated in FIG. 17.

TABLE 1 Example 1 Example 2 Example 3 Optical Substrate Si Si Si filterMultilayer Film First Material SiO SiO SiO₂ film configuration layerSecond Material Ge Ge Si layer Number of 14.5 14.5 14.5 repeating unitsFilm thickness 8432 nm 8432 nm 9780 nm Wavelength range where theaverage 4180 nm to 4330 nm 4180 nm to 4330 nm 4180 nm to 4330 nmtransmittance is 70% or more has an average has an average has anaverage from 2400 nm to 6000 nm transmittance of 81% transmittance of81% transmittance of 76% Maximum transmittance at 6000 nm to 8000 nmMaximum transmittance Maximum transmittance Maximum transmittanceAverage transmittance at 6000 nm to 8000 nm at 6000 nm to 8000 nm at6000 nm to 8000 nm at 6000 nm to 8000 nm is 51% is 51% is 62% Averagetransmittance Average transmittance Average transmittance at 6000 nm to8000 nm at 6000 nm to 8000 nm at 6000 nm to 8000 nm is 20% is 20% is 24%Infrared Semiconductor n-type InSb InSb InSb light layer of firstemitting conductive type and Semiconductor n-type AlInSb AlInSb AlInSbreceiving layer of first device conductive type Barrier layer n-typeAl_(0.22)In_(0.78)Sb Al_(0.22)In_(0.78)Sb Al_(0.22)In_(0.78)Sb Activelayer i-type Al_(0.04)In_(0.96)Sb Al_(0.04)In_(0.96)SbAl_(0.22)In_(0.96)Sb Barrier layer p-type Al_(0.22)In_(0.78)SbAl_(0.22)In_(0.78)Sb Al_(0.22)In_(0.78)Sb Semiconductor p-type AlInSbAlInSb AlInSb layer of second conductive type Example 4 Example 5Comparative Example Optical Substrate Si Si Si filter Multilayer FilmFirst Material SiO₂ SiO₂ SiO, ZnS film configuration layer SecondMaterial Si Si Ge layer Number of 14.5 14.5 50 or more repeating unitsFilm thickness 7529 nm 9780 nm >25000 nm Wavelength range where theaverage 3260 nm to 3380 nm 4180 nm to 4330 nm 4180 nm to 4330 nmtransmittance is 70 % or more has an average has an average has anaverage from 2400 nm to 6000 nm transmittance of 78% transmittance of76% transmittance of 80% Maximum transmittance at 6000 nm to 8000 nmMaximum transmittance Maximum transmittance or more Averagetransmittance at 6000 nm to 8000 nm at 6000 nm to 8000 nm at 6000 nm to8000 nm Maximum transmittance is 85% is 62% at 6000 nm to 8000 nmAverage transmittance Average transmittance is 0.5% or less at 6000 nmto 8000 nm at 6000 nm to 8000 nm Average transmittance is 52% is 24% at6000 nm to 8000 nm is 0.1% Infrared Semiconductor n-type InSb InAs InSblight layer of first emitting conductive type and Semiconductor n-typeAlInSb InAs InSb receiving layer of first device conductive type Barrierlayer n-type Al_(0.30)In_(0.70)Sb Al_(0.30)In_(0.70)AsSb None Activelayer i-type Al_(0.09)In_(0.91)Sb lnAs_(0.87)Sb_(0.13) InSb Barrierlayer p-type Al_(0.30)In_(0.70)Sb Al_(0.30)In_(0.78)AsSbAl_(0.22)In_(0.78)Sb Semiconductor p-type AlInSb InAsSb InSb layer ofsecond conductive type

According to Table 1, although Examples 1 to 5 have an optical filterwhere the film thickness is one third or less of that of ComparativeExample, they can provide equivalent performance when combined with theinfrared light emitting and receiving device. Comparative Example isdifferent from Examples 1 to 5 in that the maximum transmittance in thewavelength range of 6000 to 8000 nm is 0.5% or less, which is less than5%. In addition, Comparative Example is different from Examples 1 to 5in that the average transmittance in the wavelength range of 6000 to8000 nm is 0.1%, which is less than 2%.

According to the present disclosure, it is possible to provide a highlyaccurate NDIR gas sensor and a highly accurate optical device even usinga simplified optical filter.

In addition, the infrared light emitting device of Example 2, theinfrared light receiving device of any of Examples 1, and 3 to 5, andthe simplified filter may be combined to constitute an optical device.

1. An optical device comprising: an optical filter having a substrateand a multilayer film having a plurality of layers with differentrefractive indexes formed on at least one side of the substrate; and aninfrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; wherein the multilayer film has astructure in which a first layer and a second layer are alternatelystacked, the first layer has a refractive index of 1.2 or more and 2.5or less in a wavelength range of 2400 nm to 6000 nm, and the secondlayer has a refractive index of 3.2 or more and 4.2 or less in awavelength range of 2400 nm to 6000 nm; and the optical filter includesa wavelength range having an average transmittance of 70% or more with awidth of 50 nm or more in a wavelength range of 2400 nm to 6000 nm, andhas a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.
 2. The optical device according to claim 1, wherein theactive layer contains “Al, In, and Sb” or “In, As, and Sb.”
 3. Theoptical device according to claim 1, wherein the optical device has nosensitivity or does not emit light, in a wavelength range of 6000 nm to8000 nm.
 4. The optical device according to claim 1, wherein the opticalfilter has an average transmittance of 2% or more in a wavelength rangeof 6000 nm to 8000 nm.
 5. The optical device according to claim 1,wherein the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.05) orInAs_(y)Sb_(1-y) (0.75≤y≤0.90); and the optical filter includes awavelength range having an average transmittance of 70% or more and 95%or less with a width of 50 nm or more and 400 nm or less in a wavelengthrange of 5600 nm to 6000 nm, and has a maximum transmittance of 5% ormore in a wavelength range of 6000 nm to 8000 nm.
 6. The optical deviceaccording to claim 1, wherein the active layer contains Al_(x)In_(1-x)Sb(0.02≤x≤0.05) or InAs_(y)Sb_(1-y) (0.75≤y≤0.90); and the optical filterincludes a wavelength range having an average transmittance of 70% ormore and 95% or less with a width of 50 nm or more and 300 nm or less ina wavelength range of 5200 nm to 5500 nm, and has a maximumtransmittance of 5% or more in a wavelength range of 6000 nm to 8000 nm.7. The optical device according to claim 1, wherein the active layercontains Al_(x)In_(1-x)Sb (0.02≤x≤0.12) or InAs_(y)Sb_(1-y)(0.75≤y≤0.90); and the optical filter includes a wavelength range havingan average transmittance of 70% or more and 95% or less with a width of50 nm or more and 300 nm or less in a wavelength range of 4500 nm to4800 nm, and has a maximum transmittance of 5% or more in a wavelengthrange of 6000 nm to 8000 nm.
 8. The optical device according to claim 1,wherein the active layer contains Al_(x)In_(1-x)Sb (0.02≤x≤0.12) orInAs_(y)Sb_(1-y) (0.75≤y≤0.90); and the optical filter includes awavelength range having an average transmittance of 70% or more and 95%or less with a width of 50 nm or more and 150 nm or less in a wavelengthrange of 4200 nm to 4350 nm, and has a maximum transmittance of 5% ormore in a wavelength range of 6000 nm to 8000 nm.
 9. The optical deviceaccording to claim 1, wherein the active layer contains Al_(x)In_(1-x)Sb(0.04≤x≤0.14) or InAs_(y)Sb_(1-y) (0.80≤y≤1); and the optical filterincludes a wavelength range having an average transmittance of 70% ormore and 95% or less with a width of 50 nm or more and 300 nm or less ina wavelength range of 3300 nm to 3600 nm, and has a maximumtransmittance of 5% or more in a wavelength range of 6000 nm to 8000 nm.10. The optical device according to claim 1, wherein the active layercontains Al_(x)In_(1-x)Sb (0.04≤x≤0.14) or InAs_(y)Sb_(1-y) (0.80≤y≤1);and the optical filter includes a wavelength range having an averagetransmittance of 70% or more and 95% or less with a width of 50 nm ormore and 200 nm or less in a wavelength range of 3200 nm to 3400 nm, andhas a maximum transmittance of 5% or more in a wavelength range of 6000nm to 8000 nm.
 11. The optical device according to claim 1, wherein theactive layer contains Al_(x)In_(1-x)Sb (0.08≤x≤0.20) or InAs_(y)Sb_(1-y)(0.80≤y≤1); and the optical filter includes a wavelength range having anaverage transmittance of 70% or more and 95% or less with a width of 50nm or more and 400 nm or less in a wavelength range of 2400 nm to 2800nm, and has a maximum transmittance of 5% or more in a wavelength rangeof 6000 nm to 8000 nm.
 12. The optical device according to claim 1,wherein a film thickness of the multilayer film is 5000 nm or more and25000 nm or less.
 13. The optical device according to claim 1, whereinthe number of times the first layer and the second layer are alternatelystacked in the multilayer film is 10 or more and 60 or less.
 14. Theoptical device according to claim 1, wherein the first layer contains atleast one selected from the group consisting of SiO, SiO₂, TiO₂ and ZnS.15. The optical device according to claim 1, wherein the second layercontains at least one selected from the group consisting of Si and Ge.16. An optical device comprising: an optical filter having a substrateand a multilayer film having a plurality of layers with differentrefractive indexes formed on at least one side of the substrate; and aninfrared light emitting and receiving device having a semiconductorlayer of a first conductive type, an active layer, and a semiconductorlayer of a second conductive type; wherein the multilayer film has astructure in which a first layer and a second layer are alternatelystacked, the first layer contains at least one selected from the groupconsisting of SiO, SiO₂, TiO₂ and ZnS, and the second layer contains atleast one selected from the group consisting of Si and Ge; and theoptical filter includes a wavelength range having an averagetransmittance of 70% or more with a width of 50 nm or more in awavelength range of 2400 nm to 6000 nm, and has a maximum transmittanceof 5% or more in a wavelength range of 6000 nm to 8000 nm.